1. Field of the Invention
The present invention relates to an inkjet printing apparatus that performs printing by externally applying an energy to ink to eject it onto the print medium.
2. Description of the Related Art
There is a growing need in recent years for the inkjet printing apparatus to stably eject smaller ink droplets precisely in a desired direction in order to realize a faster printing of highly defined images. A popular method currently available to meet this requirement uses a print head mounted in a carriage and having arrays of nozzles with smaller orifices and causes the print head to eject ink from the orifices as it is scanned over a print medium at high speed in a forward and a backward direction. When the printing operation is performed in both directions—forward and backward—printed images may have variations in color because the forward and backward direction printing have the opposite carriage movements or more precisely the order in which different color inks land on the print medium is reversed between the forward and backward direction printing.
FIG. 15A and FIG. 15B show a conventional print head, FIG. 15A being a front view and FIG. 15B a side view. A method has been known which arranges laterally symmetrically nozzle arrays for ejecting magenta and cyan inks to prevent color differences from appearing during printing. This print head can have the same order of color ink landing no matter in which direction—forward and backward—the print head is moving, thus minimizing color differences that would otherwise be caused by a difference in the color ink landing order. As a result, the aforementioned color difference is prevented from appearing even if an image is formed with an odd number of passes. (Japanese Patent Laid-Open No. H7-112534 (1995)).
The print head shown in FIG. 15A has a black chip 010, which is longer than a color chip 011, arranged by the side of the color chip 011 to improve throughput when printing a document with only a black ink. A printing method using such a print head has been known which chooses from among three print modes—one using only the black chip 010, one using only the color chip 011 and one using both of them—according to the kind of material to be printed.
However, although the nozzle arrays in the color chip 011 are arranged symmetrical in the chip in the forward and backward directions, the positioning of the black chip 010, which is placed on only one side of the color chip 011, makes the print head as a whole unsymmetrical in the forward and backward directions. So, the state of air currents flowing between the inkjet print head 014 and the print medium 013 during the printing operation differs between the forward direction printing and the backward direction printing. Therefore, even if the nozzle arrays of the same ink colors are arranged laterally symmetrical as shown in FIG. 15A, the landing positions of main droplets and satellites differ between the forward direction printing and the backward direction printing, causing unevenness in the printed state of an image on the print medium, degrading the image quality. This will be detailed in the following.
FIG. 16A to FIG. 16D show the states of air currents produced between the print head 014 and the print medium 013 during the forward direction printing. The state of air current when the print head 014 is scanned in the forward direction is shown in FIG. 16A and FIG. 16B; and the state of air current when the print head is scanned in the backward direction is shown in FIG. 16C and FIG. 16D. FIG. 16A to FIG. 16D schematically show air current velocity distributions 060 between the print head 014 and the print medium 013. Either in the forward and backward direction, it is seen that the velocity of air currents flowing from the front beneath the nozzle array in the rear is slower than that beneath the nozzle array in front with respect to the direction of carriage movement. This is because the air that has flown in between the print head 014 and the print medium 013 escapes from both sides of the print head 014 as it travels downstream in the direction of its movement, as shown in FIG. 16B and FIG. 16D.
FIG. 17A and FIG. 17B show main droplets and satellites that have landed on the print medium 013, FIG. 17A schematically representing the landing positions during the forward direction printing and FIG. 17B the landing positions during the backward direction printing. Whether the print head is moving in the forward or backward direction, the satellite 022 has a slower ejection speed than the main droplet, so that the satellite 022 lands on the print medium at a position beyond that of the main droplet with respect to the direction of movement of the print head. However, the distance a1 between the main droplet 021 and the satellite 022 of an ink droplet ejected from a nozzle array in the rear with respect to the direction of movement of the print head is greater than the distance b1 between the main droplet 021 and the satellite 022 of an ink droplet ejected from a nozzle array in front because the velocity of air currents flowing from the front beneath the rear nozzle array is slower than that beneath the front nozzle array.
The satellite 022 with a slower ejection speed than that of the main droplet 021 is pushed backward while flying by an air current from the front with respect to the direction of print head movement. At this time, since the air currents from the front are slower beneath the nozzle array in the rear, the satellite 022 of an ink droplet ejected from the rear nozzle array is hardly affected by the air current and the distance the satellite 022 is pushed backward is therefore reduced. The similar result is observed also in the relationship between the main droplet 021 and the satellite 022 on the print medium 013 when a backward direction printing is performed, as shown in FIG. 17B. That is, the distance b1′ between the main droplet 021 and the satellite 022 of an ink droplet ejected from a nozzle array in the rear with respect to the direction of movement of the print head is greater than the distance a1′ between the main droplet 021 and the satellite 022 of an ink droplet ejected from a nozzle array in front.
Here, as shown in FIG. 15B, the conventional inkjet print head 014 has the distance from its front end to a front nozzle array with respect to the direction of movement of the print head differ between the forward direction printing and the backward direction printing, and also has the distance from its rear end to a rear nozzle array differ between the forward direction printing and the backward direction printing. Let us take a cyan nozzle array 002 as an example and consider the distance from the front end of the print head and the nozzles of the cyan nozzle array. The distances to the front nozzles with respect to the direction of movement of the print head are s1′≠s1 and those to the rear nozzles are t1′≠t1. This means that the distance between the main droplet 021 and the satellite 022 formed on the print medium 013 differs between the forward direction printing and the backward direction printing. More precisely, in FIG. 17A and FIG. 17B, as for the distance to the front nozzle array with respect to the direction of movement of the print head, b1≠a1′; and as for the distance to the rear nozzle array with respect to the direction of movement of the print head, a1≠b1′.
What has been described above contributes to the problem with the conventional printing apparatus that the degree to which the surface of the print medium 013 is covered with dots differs between the forward direction printing and the backward direction printing, causing grayscale level variations in printed images and therefore uneven print quality.